1. Field of the Invention
The present invention relates to loop type coil parts used for various thin electrical instruments.
2. Description of the Related Technology
Loop type coil parts, such as an inductor, a transformer and a common mode choking coil, used for various electrical instruments are generally such that a coil lead is wound around a spool of a drum type core, and an end of the coil lead is soldered to a terminal electrode provided on a bottom surface of a flange which is in contact with a mounting side. Specifically, as disclosed in JP-UM-A-3-083909 (FIG. 7), there is known a surface mount coil part 1 in which a groove 5 is provided in a bottom surface of a flange 4 of a drum type core which is in contact with a mounting side of a circuit board, terminal electrodes 6 are provided at both sides of the groove of the bottom surface, an end 2a of a coil lead 2 wound around a spool of the drum type core is inserted in the groove 5 and is soldered to the terminal electrodes 6 of the bottom surface. As shown in FIG. 8, the coil part 1 as stated above is mounted on a land electrode 7a previously printed with a solder paste on the mounting side of a circuit board 7, and is solder mounted by reflow soldering or the like. At this time, the size of a fillet 5 formed between the land electrode 7a of the circuit board 7 and the terminal electrode 6 of the coil part 1 is observed using an automatic visual-inspection equipment or the like, so that confirmation of quality of a soldering state between the terminal electrode 6 of the coil part 1 and the land electrode 7a of the circuit board 7 is performed. Although not shown in the drawings, JP-A-7-115023 proposes a coil part in which in order to facilitate the inspection of soldering properties by using the fillet, a groove is provided in the bottom surface of a flange of a drum type core which is in contact with the mounting side of a circuit board, an electrode continuous with an inner wall of the groove, a bottom surface adjacent thereto, and a side surface of the flange in contact with the bottom surface is provided, and the fillet is formed on the side surface.
In recent years, there is increasing a tendency that thin electrical instruments are used in consideration of space-saving and portability, and thin coil parts used for these electrical instruments are desired. However, in the surface mount coil part of the related art disclosed in JP-UM-A-3-083909, since only the fillet corresponding to the sectional area of the groove of the flange is formed, in order to obtain a large fillet, the thickness of the flange of the drum type core is made thick and a deep and large groove must be provided, and the height size of the coil part becomes high by that, and accordingly, it has been difficult to obtain a low back type coil. Besides, when the thickness size of the flange is made small in order to obtain the low back type coil part, it has been difficult to provide an electrode for fillet confirmation with a sufficient area on the side surface of the flange like the coil part of the structure disclosed in JP-A-7-115023.